The two most common types of polymerization initiators for ophthalmic device materials are thermal initiators and photoinitiators. Typical thermal initiators, including free radical initiators such as peroxides, initiate polymerization as temperature is increased. In some cases, two or three temperature tiers are involved such that curing involves a schedule of temperature/time combinations. Thermal initiation generally requires holding the monomer composition at elevated temperatures for lengthy periods of time. Total cure times of twenty-four hours are not unusual. See, for example, U.S. Pat. No. 5,290,892.
Photoinitiators generally offer the advantage of relatively short cure times and, unlike thermal initiators, can be used at ambient conditions, eliminating the need for high-temperature equipment or special ovens. Photoinitiators are activated by radiation of one or more specified wavelengths, rather than heat. Photoinitiation of ophthalmic lens materials is known. See, for example, U.S. Pat. No. 5,290,892.
The most common types of photoinitiators known or used for curing ophthalmic lens polymers are probably UV-sensitive photoinitiators. UV-sensitive photoinitiators are, however, generally not suitable for use with lens materials that contain a UV-absorbing chromophore. UV-absorbing chromophores present in an ophthalmic lens composition can interfere with the ability of UV-sensitive photoinitiators to efficiently cure the composition. Today, UV-absorbing chromophores are frequently incorporated in ophthalmic lens materials in order to reduce or block UV light from reaching the retina. Although methods are known for temporarily "blocking" UV absorbing chromophores during processing, thereby preventing interference with a UV-initiator, these methods require that the UV-absorber be "un-blocked" after the composition is cured. The UV chromophore can be "un-blocked" by either chemical or thermal means. "Un-blocking" is generally complicated and can add 4-6 hours to processing times, offsetting some or all of the time advantages offered by photoinitators.
In addition to UV-sensitive photoinitiators, visible-light initiators are also known. For example, U.S. Pat. No. 5,224,957 discloses photopolymerizable compositions useful in forming an intraocular lens in situ. The compositions are delivered into the natural lens capsule or a thin plastic shell substitute and then polymerized. The reference compositions contain 90-99.99% by weight of at least one polyfunctional acrylic and/or methacrylic acid ester. Suitable acid esters include bisphenol A or bishydroxypolyalkoxy bisphenol A derivatives lengthened with ethylene oxide or propylene oxide. The compositions of the '957 patent are cured using photoinitiators which absorb light in the range 400-500 nm. Suitable initiators include alpha-diketones, in particular camphorquinone, benzil and phenanthrene quinone, and mono and bisacylphosphine oxides.
International Patent Application Publication No. WO 96/28762 also discloses photocurable compositions comprising acrylic components. The compositions contain specified amounts of di(meth)acrylates, poly(meth)acrylates, urethane(meth)acrylates, and oligomeric di(meth)acrylates based on bisphenol A or bisphenol F. The photoinitiator may be "any photoinitiator which forms free radicals when irradiated suitably." Suitable classes include benzoin ethers; acetophenones; benzil; anthraquinones; benzoylphosphine oxides (e.g., 2,4,6-trimethylbenzoyldiphenylphosphine oxide); benzophenones. Photoinitiators particularly suitable for use with argon ion lasers include 2,4,6-trimethylbenzoyldiphenylphosphine oxide.
Some ophthalmic devices are obtained by a monomer cast polymerization method. In such a method, the monomer solution is cast directly into a mold of desired shape and then polymerized or cured, followed by any machining or polishing, etc. See, for example, U.S. Pat. Nos. 4,921,205 and 5,290,892.
In other cases, ophthalmic device materials are formed by first preparing a "prepolymer" or partially cured material, followed by further curing. See, for example, U.S. Pat. No. 5,374,663 describing a prepolymer process for producing a U.V. absorber-containing intraocular lens material in which a monomer solution comprising a lens-forming monomer, an U.V. absorber and a polymerization initiator is introduced into a reactor and heated for a length of time and at a temperature sufficient to obtain a prepolymer of high viscosity. Thereafter, the prepolymer is filtered, cast into a cell or mold and further heated for a time at a temperature sufficient to obtain a transparent lens material.
According to the '663 patent, the prepolymer process has the advantage that the prepolymer scarcely leaks out of the cell or mold because of its high viscosity, and that the degree of shrinkage in the step of obtaining a lens material from the prepolymer is small. On the other hand, the prepolymer process has some problems as well, including (i) the control of the polymerization degree and viscosity of the prepolymer obtained in the first polymerization step, and (ii) when a cross-linking monomer is contained in the material, an insoluble polymer is formed in the prepolymer step, making any filtration treatment difficult or impossible, and the polymer produced after the further curing step becomes "non-uniform."